Wiktionary
defective verb
n. (context grammar English) A verb with an incomplete conjugation; for example, one that can only be conjugated in certain persons and numbers.
Wikipedia
Defective verb
In linguistics, a defective verb is a verb with an incomplete conjugation, or one which cannot be used in some other way as normal verbs can. Defective verbs cannot be conjugated in certain tenses, aspects, or moods.